Synoptic features on moisture field and shallow convection in the subtropical high area around Japan in early july of 1988

The characteristics of the subtropical high around Kyushu as the water transport system toward the Baiu front was examined. Two periods in early July of 1988 were selected in this study (the period (I): 6 to 10 July and (II): 11 to 15 July) when the special rawinsonde upper-air observation was made there. In period (I) the northward transport of moisture across the ridge line of the subtropical high was very weak. The air was very dry, especially above the 900 mb level. In period (II) the enhancement of the northward transport of moisture was accompanied by a strong low-level southerly wind. It is noted that the specific humidity above the 900 mb level (~ above the planetary boundary layer) also increased then, which results in a large amount of precipitable water in the subtropical high area in period (II). In period (I) the amount of low-level clouds is very small in the subtropical high area, while it attains 20 to 60 % due to the activation of shallow cumulus convection there in period (II). Furthermore, the maximum phases of diurnal variations of the amount of low-level clouds, specific humidity and relative humidity were in coincidence with each other over the sea. The moisture content did not decrease along the low-level southerly wind in the subtropical high area in period (II). Although the present study does not examine the moisture budget there, the shallow cumulus convection might play an important role in sustaining the large moisture content, especially above the planetary boundary layer, in the subtropical high area to the south of the Baiu front.